Center for Gene Regulation in Health and Disease, Department of Biological, Geological, and Environmental Sciences, College of Sciences and Health Professions, Cleveland State University, 2121 Euclid Avenue, Cleveland, OH 44115, USA.
Case Comprehensive Cancer Center, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA.
Nucleic Acids Res. 2021 Jun 4;49(10):5637-5653. doi: 10.1093/nar/gkab401.
Telomere repeat-containing RNA (TERRA) has been identified in multiple organisms including Trypanosoma brucei, a protozoan parasite that causes human African trypanosomiasis. T. brucei regularly switches its major surface antigen, VSG, to evade the host immune response. VSG is expressed exclusively from subtelomeric expression sites, and we have shown that telomere proteins play important roles in the regulation of VSG silencing and switching. In this study, we identify several unique features of TERRA and telomere biology in T. brucei. First, the number of TERRA foci is cell cycle-regulated and influenced by TbTRF, the duplex telomere DNA binding factor in T. brucei. Second, TERRA is transcribed by RNA polymerase I mainly from a single telomere downstream of the active VSG. Third, TbTRF binds TERRA through its C-terminal Myb domain, which also has the duplex DNA binding activity, in a sequence-specific manner and suppresses the TERRA level without affecting its half-life. Finally, levels of the telomeric R-loop and telomere DNA damage were increased upon TbTRF depletion. Overexpression of an ectopic allele of RNase H1 that resolves the R-loop structure in TbTRF RNAi cells can partially suppress these phenotypes, revealing an underlying mechanism of how TbTRF helps maintain telomere integrity.
端粒重复 RNA（TERRA）已在多种生物中被发现，包括引起非洲人类锥虫病的原生动物寄生虫——布氏锥虫。布氏锥虫经常切换其主要表面抗原 VSG，以逃避宿主免疫反应。VSG 仅从端粒外显子表达位点表达，我们已经表明端粒蛋白在 VSG 沉默和切换的调节中发挥重要作用。在这项研究中，我们鉴定了布氏锥虫中 TERRA 和端粒生物学的几个独特特征。首先，TERRA 焦点的数量受细胞周期调控，并受 TbTRF 影响，TbTRF 是布氏锥虫中双链端粒 DNA 结合因子。其次，TERRA 主要由 RNA 聚合酶 I 转录，主要来自活跃的 VSG 下游的单个端粒。第三，TbTRF 通过其 C 末端 Myb 结构域与 TERRA 结合，该结构域也具有双链 DNA 结合活性，以序列特异性方式结合并抑制 TERRA 水平而不影响其半衰期。最后，在 TbTRF 耗尽时，端粒 R 环和端粒 DNA 损伤的水平增加。在 TbTRF RNAi 细胞中过表达可以解决 R 环结构的异位 RNase H1 等位基因可以部分抑制这些表型，揭示了 TbTRF 如何帮助维持端粒完整性的潜在机制。
